To produce a lens with a colored layer, a coloring material is prepared as preliminary preparation for molding of a lens body by processing a film-like or sheet-like material into a shape of a certain outline and a certain curvature. Then, the coloring material is provided with a colored layer having a certain gradation pattern, for example, and is placed in a position in a mold for injection molding, thereby molding the lens body into a certain lens shape.
There has been an example intended to provide a method of manufacturing a polarizing lens that makes it possible to ensure uniformity of a product lens stably by allowing a polarizing film used as the aforementioned coloring material to be placed reliably in a position in a mold and by making a molded lens have uniform density so it is less likely distorted.
The aforementioned example is a method of manufacturing a plastic polarizing lens using a split mold having a mold surface to form a lens curvature. At least a part of the mold surface to which a polarizing film is inserted is configured as a movable part with which a recess can be generated in response to upward and downward movement of the movable part. The polarizing film molded into a shape corresponding to the movable part is hot pressed into a curved shape conforming to the lens curvature. Then, the movable part is moved downward from a side of the lens curvature to generate the recess. The curved polarizing film is inserted into the recess and the mold is closed. Next, while a molten resin for molding of a lens is poured in a runner continuous with the mold, the molten resin is injected into a cavity of the mold through a gate.
However, if the aforementioned manufacturing method is applied to a piano lens as a thin lens, assuming the thickness of a lens as a finished article is 2 mm and the thickness of a coloring material sheet used in the preliminary preparation is 0.6 mm, the molten resin is to be flown into a gap of 1.4 mm of the mold during injection molding. An injection speed should be increased if a lens is to be molded by injecting the molten resin into such a narrow gap.
In response to increase in the injection speed, the increased speed and the heat of the molten resin act together to melt a coloring material surface while the coloring material surface is pressed with the molten resin. This moves or peels a colored layer provided on this surface by staining, resulting in the problem of uneven coloring or peeling of the colored layer. Examples of such a problem are shown in FIGS. 5A, 5B, and 5C. In these examples, a relationship of an injection direction relative to a shading direction of gradation of a coloring material is determined such that an edge of a darker side or a right or left edge agrees with an upstream side of the injection direction.
Molding a lens with the polarizing film 2 inserted in the recess H of the mold like in Patent Literature causes a problem in that a molding burr pointed toward a convex side of the lens occurs at a peripheral portion of a prototype Lb of the molded lens corresponding to a boundary portion of the movable part 12 of the split mold 1.